(1) Background: In children, SARS-CoV-2 infection is mostly accompanied by mild COVID-19 symptoms. However, multisystem inflammatory syndrome (MIS-C) and long-term sequelae are often severe complications. Therefore, the protection of the pediatric population against SARS-CoV-2 with effective vaccines is particularly important. Here, we compare the humoral and cellular immune responses elicited in children (n = 15, aged 5–11 years) vaccinated with the RBD-based vaccines SOBERANA^® 02 and SOBERANA^® Plus combined in a heterologous scheme with those from children (n = 10, aged 4–11 years) who recovered from mild symptomatic COVID-19. (2) Methods: Blood samples were taken 14 days after the last dose for vaccinated children and 45–60 days after the infection diagnosis for COVID-19 recovered children. Anti-RBD IgG and ACE2-RBD inhibition were assessed by ELISA; IgA, cytokines, and cytotoxic-related proteins were determined by multiplex assays. Total B and T cell subpopulations and IFN-γ release were measured by multiparametric flow cytometry using a large panel of antibodies after in vitro stimulation with S1 peptides. (3) Results: Significant higher levels of specific anti-RBD IgG and IgA and ACE2-RBD inhibition capacity were found in vaccinated children in comparison to COVID-19 recovered children. Th1-like and Th2-like CD4⁺ T cells were also significantly higher in vaccinated subjects. IFN-γ secretion was higher in central memory CD4⁺ T cells of COVID-19 recovered children, but no differences between both groups were found in the CD4⁺ and CD8⁺ T cell effector, terminal effector, and naïve T cell subpopulations. In contrast to low levels of IL-4, high levels of IL-2, IL-6, IFN-γ, and IL-10 suggest a predominant Th1 cell polarization. Cytotoxic-related proteins granzyme A and B, perforin, and granulin were also found in the supernatant after S1 stimulation in both vaccinated and recovered children. (4) Conclusions: Vaccination with the heterologous scheme of SOBERANA^® 02/SOBERANA^® Plus induces a stronger antibody and cellular immune response compared to natural infections in young children.

(1) Background: SARS-CoV-2 infection is mostly accompanied by mild COVID-19 symptoms in children. However, the multisystem inflammatory syndrome (MIS-C) and long-term sequelae are often severe complications. Therefore, the protection of the pediatric population against SARS-CoV-2 with effective vaccines is particularly important. Here we compare the humoral and cellular immune responses elicited in children (n=15) aged 5-11 years vaccinated with RBD-based vaccines combined in a heterologous scheme of SOBERANA® 02 and SOBERANA® Plus with those from children (n=10) aged 4-11 years who recovered from mild symptomatic COVID-19. (2) Methods: Blood samples were taken 14 days after last dose for vaccinated and 45-60 days after the infection diagnosis for COVID-19 recovered children. Anti-RBD IgG and ACE2-RBD inhibition were assessed by ELISA; IgA, cytokine and cytotoxic related proteins profile were determined by multiplex assays. Total B and T cell subpopulations and IFN-γ release were measured by multiparametric flow cytometry using a large panel of antibodies after in vitro stimulation with S1 peptides. (3) Results: Significant higher levels of specific anti-RBD IgG and IgA and ACE2-RBD inhibition capacity were found in vaccinated children in comparison to COVID-19 recovered children. Th1-like and Th2-like CD4+ T cells were also significantly higher in vaccinated subjects. IFN-γ secretion were higher in central memory CD4+ T cells of COVID-19 recovered children, but no differences between both groups were found in CD4+ and CD8+ T cells effector, terminal and naïve T cell subpopulations. High levels of IL-2, IL-6, IFN-γ and IL-10 in contrast to low levels of IL-4 suggesting a predominant Th1 cell polarization. Cytotoxic-related proteins granzyme A and B, perforin and granulin were also found in the supernatant after S1 stimulation in both vaccinated and recovered children. (4) Conclusions: Vaccination with the heterologous scheme of SOBERANA® 02/ SOBERANA® Plus induces strong antibody and cellular immune response compared to natural infections of young children.

Background: With the emergence of SARS-CoV-2 Omicron sub-variants exhibiting increased transmissibility and immune escape, booster immunization is recommended. Ideally, vaccination across all age groups, including children and adolescents, is critical to control viral spread and reduce variant emergence. The heterologous scheme consisting of two doses of SOBERANA^® 02 followed by a third dose of SOBERANA^® Plus, which are recombinant protein subunit vaccines constructed from the ancestral RBD, has proven safety, immunogenicity, and effectiveness in pediatric populations as primary series. This study evaluated the safety and immunogenicity of a SOBERANA^® Plus booster dose administered six months after primary vaccination in individuals aged 3–18 years. Methods: In this follow-up analysis of a phase I/II trial, 244 participants received the booster. Safety was monitored via active surveillance at 1 h, 24 h, and over 28 days post-vaccination. Humoral responses were assessed 28 days post-booster. Antibody responses to the SARS-CoV-2 nucleocapsid (N) protein were assessed in all collected serum samples. Results: Adverse events occurred in 18% of participants, predominantly local (85.2%) versus systemic (14.8%); no serious or severe adverse events were reported. All humoral response parameters increased significantly post-booster, including neutralizing antibodies against D614G (24.7-fold increase) and Omicron BA.1 (55.9-fold increase), with similar responses in N-negative and N-positive individuals. Importantly, cross-neutralizing activity against recent Omicron sub-variants (XBB.1.5 and EG.5.1) was also detected. Conclusions: A SOBERANA^® Plus booster is safe and significantly enhances cross-neutralizing immunity against evolving Omicron sub-variants in children and adolescents. These results highlight the potential of first-generation RBD-based vaccines to maintain broad immunity despite viral evolution.

Background: With the emergence of SARS-CoV-2 Omicron sub-variants exhibiting in-creased transmissibility and immune escape, booster immunization is recommended. Ideally, vaccination across all age groups, including children and adolescents, is critical to control viral spread and reduce variant emergence. The heterologous three-dose regimen of SOBERANA® 02 and SOBERANA® Plus—protein subunit vaccines based on the orig-inal RBD—has proven safe, immunogenic, and effective in pediatric populations as a primary series. This study evaluated the safety and immunogenicity of a SOBERANA® Plus booster dose administered six months after primary vaccination in individuals aged 3–18 years. Methods: In this follow-up analysis of a phase I/II trial, 244 participants re-ceived the SOBERANA® Plus booster. Safety was monitored via active surveillance at 1 hour, 24 hours, and over 28 days post-vaccination. Humoral responses were assessed 28 days post-booster. All serum samples were tested for antibodies against the SARS-CoV-2 nucleocapsid (N) protein. Results: Adverse events occurred in 18% of participants, pre-dominantly local (85.2%) versus systemic (14.8%); no serious or severe adverse events were reported. All humoral response parameters increased significantly post-booster, in-cluding neutralizing antibodies against D614G and Omicron BA.1, with similar respons-es in N-negative and N-positive individuals. Importantly, cross-neutralizing activity against recent Omicron sub-variants (XBB.1.5 and EG.5.1) was also detected. Conclusions: A SOBERANA® Plus booster is safe and significantly enhances cross-reactive neutraliz-ing immunity against evolving Omicron sub-variants in children and adolescents. These results highlight the potential of first-generation RBD-based vaccines to sustain broad immune protection despite viral evolution.